The present invention disclosed herein relates to an electrolyte including a compound having a functional group which can react with a side reaction site of a negative electrode material in a secondary battery and a functional group which can react with moisture, and to a secondary battery with improved storage performance at high temperatures by including the electrolyte to prevent the degradation of the performance of the battery.
A lithium secondary battery, widely used as a power source of a laptop computer, a camcorder, a mobile phone, etc., consists of a cathode including a lithium metal complex oxide which is capable of intercalating and disintercalating lithium ions or sulfur, an anode containing a carbon material or a metal lithium, etc., and an electrolyte in which an appropriate amount of a lithium salt is dissolved in a mixed organic solvent.
Lithium ions released from the cathode active material such as lithium metal oxide during an initial charging process of a lithium secondary battery move towards the anode active material such as graphite and then are intercalated between the layers of the anode active material. At this time, due to the high reactivity of lithium, the electrolyte reacts with carbon of the anode active material on the surface of the anode active material such as graphite, thereby generating compounds such as Li2CO3, Li2O and LiOH. These compounds form a kind of Solid Electrolyte Interface (SEI) film on the surface of the anode active material such as graphite.
The SEI film serves as an ion tunnel, which allows only lithium ions to pass. Due to the ion tunnel effects, the SEI film prevents an organic solvent having a high molecular weight, which moves together with lithium ions in the electrolyte from being intercalated into layers of the anode active material and thus breaking down the anode structure. Thus, since the electrolyte is not contacted with the anode active material, the electrolyte is not decomposed, and also the amount of lithium ions in the electrolyte is reversibly maintained, thereby ensuring stable charging/discharging.
However, if a battery is left at a high temperature in a fully charged state, the SEI film is slowly broken down due to increased electrochemical energy and thermal energy over time. As a result, side reactions continuously occur between the exposed surface of the anode and surrounding electrolyte. Due to continuous gas generation at this time, an internal pressure of the battery is increased, thereby increasing thicknesses of a prismatic battery and a pouch battery, and this may cause problems in electronics such as mobile phones and laptop computers. That is, the battery has poor stability at high temperatures.
Therefore, various studies to enhance the stability of a battery at high temperatures have been conducted. In order to inhibit the internal pressure of a battery from increasing, studies on a method of adding an additive to the electrolyte so as to change the phase of the SEI film forming reaction are representative among them. For example, Japanese Patent Application Laid-Open Publication No. H07-176323 discloses an electrolyte to which CO2 is added. Japanese Patent Application Laid-Open Publication No. H07-320779 discloses a technology that a sulfide-based compound is added to an electrolyte to inhibit the decomposition of the electrolyte. Japanese Patent Application Laid-Open Publication No. H09-73918 discloses a technology that diphenyl picrylhydrazyl (DPPH) is added to enhance the stability of a battery at high temperatures. Japanese Patent Application Laid-Open Publication No. H08-321313 discloses a technology that a certain compound is added to enhance the charge/discharge cycle life of a battery.